Many known anti-lock devices operate by cyclically increasing and decreasing the braking force exerted on the wheels so that a wheel having a tendency to lock is permitted to re-accelerate back toward a speed corresponding to the speed of the vehicle. This is typically achieved by control valves alternately allowing fluid to flow out of and then into the brake cylinder to first lower and then raise the brake pressure in the brake system. Some anti-lock braking systems (ABS) employ a pump-back scheme where fluid is dumped from the wheel cylinder to a local accumulator and the same hydraulic fluid is re-supplied from the local accumulator to the brake pad actuators.
Most of such anti-lock braking systems are further capable of operating in a traction control mode (sometimes called “dynamic rear proportioning”). Traction control and anti-lock operation are both responses to aberrant vehicle wheel behavior. A traction control function is established by detecting conditions where the rotational speed of a first powered wheel substantially exceeds that of a second powered wheel. To provide a power balance in the operation of the vehicle, a braking force is applied to the powered wheel rotating at a higher speed to effectively transfer driving torque back to the other wheel that has better traction. Many anti-lock systems having such a traction control feature employ a motor and hydraulic pump or pumps which operate independent of the service braking system to supply fluid from a local accumulator to brake the wheel which has lost traction. The same local accumulator may be utilized during either mode of operation.
With additional sensors, such as accelerometers, monitoring a plurality of additional vehicle operating parameters, e.g., vehicle yaw, electronic stability programs (ESP) are providing enhanced vehicle safety. Like anti-lock and traction control, the ESP systems utilize hydraulic pumping units with one or more fluid accumulators responsive to the monitored parameters to selectively brake certain wheels and maintain vehicle control.
In all these systems, it is desirable to have an immediately available source of hydraulic pressure to selectively apply a corrective braking force in response to certain sensed anomalies and to provide a temporary storage location to which fluid may be vented. With new designs and additional features, it becomes increasingly important to minimize the size and weight of the pump/reservoir units and to adapt those units to a variety of specific configurations. For example, pistons of various axial lengths may be employed in a common diameter accumulator. Moreover, ease and economy of manufacture are important. Prior designs do not allow spacing the seal ring at an advantageous depth, therefore, the piston length and housing depth is unnecessarily long. The used material is not optimized.
The reservoir bore for ESP brake systems is sealed off by an elastomer seal ring and a staked, crimped or orbital riveted-in closing cover. The reservoir is typically a stroke piston design. The seal ring requires a groove for its retention. This groove is formed by a step in the reservoir bore that is machined into the reservoir bore, and the lip or rim of the closing cover. Due to the reservoir size it is highly desirable to be able to place the seal ring at any desirable depth in the reservoir bore to optimize material, stroke and design and therefore cost. It is also desirable that only one component to be used for the assembly. Two components, a cover and a spacer ring, are possible but would make the design and assembly unnecessary complicated and complex.